1. Field of the Invention
The invention relates generally to hardfacing compositions. More specifically, the present invention relates to hardfacing compositions for use on milled tooth drill bits.
2. Background Art
Drill bits used to drill wellbores through earth formations generally are made within one of two broad categories of bit structures. Drill bits in the first category are generally known as “fixed cutter” or “drag” bits, which usually include a bit body formed from steel or another high strength material and a plurality of cutting elements disposed at selected positions about the bit body. The cutting elements may be formed from any one or combination of hard or superhard materials, including, for example, natural or synthetic diamond, boron nitride, and tungsten carbide.
Drill bits of the second category are typically referred to as “roller cone” bits, which include a bit body having one or more roller cones rotatably mounted to the bit body. The bit body is typically formed from steel or another high strength material. The roller cones are also typically formed from steel or other high strength material and include a plurality of cutting elements disposed at selected positions about the cones. The cutting elements may be formed from the same base material as is the cone. These bits are typically referred to as “milled tooth” bits. Other roller cone bits include “insert” cutting elements that are press (interference) fit into holes formed and/or machined into the roller cones. The inserts may be formed from, for example, tungsten carbide, natural or synthetic diamond, boron nitride, or any one or combination of hard or superhard materials.
Milled tooth bits include one or more roller cones rotatably mounted to a bit body. The one or more roller cones are typically made from steel and include a plurality of teeth formed integrally with the material from which the roller cones are made. Typically, a hardfacing material is applied, such as by arc or gas welding, to the exterior surface of the teeth to improve the wear resistance of the teeth. The hardfacing material typically includes one or more metal carbides, which are bonded to the steel teeth by a metal alloy (“binder alloy”). In effect, the carbide particles are suspended in a matrix of metal forming a layer on the surface. The carbide particles give the hardfacing material hardness and wear resistance, while the matrix metal provides fracture toughness to the hardfacing.
Many factors affect the durability of a hardfacing composition in a particular application. These factors include the chemical composition and physical structure (size and shape) of the carbides, the chemical composition and microstructure of the matrix metal or alloy, and the relative proportions of the carbide materials to one another and to the matrix metal or alloy.
The metal carbide most commonly used in hardfacing is tungsten carbide. Small amounts of tantalum carbide and titanium carbide may also be present in such material, although these other carbides are considered to be deleterious. It is quite common to refer to the material in the hardfacing merely as “carbide” without characterizing it as tungsten carbide. It should be understood that as used herein, “carbide” generally refers to tungsten carbide.
Many different types of tungsten carbides are known based on their different chemical compositions and physical structure. Three types of tungsten carbide commonly used in hardfacing drill bits are cast tungsten carbide, macro-crystalline tungsten carbide, and cemented tungsten carbide (also known as sintered tungsten carbide), the most common among these being crushed cast carbide.
Tungsten generally forms two carbides, monotungsten carbide (WC) and ditungsten carbide (W2C). Tungsten carbide may also exist as a mixture of these two forms with any proportion between the two. Cast carbide is a eutectic mixture of the WC and W2C compounds, and as such the carbon content in cast carbide is sub-stoichiometric, (i.e., it has less carbon than the more desirable WC form of tungsten carbide). Cast carbide is prepared by freezing carbide from a molten state and crushing and comminuting the resultant particles to the desired particle size.
Macro-crystalline tungsten carbide is essentially stoichiometric WC in the form of single crystals. While most of the macro-crystalline tungsten carbide is in the form of single crystals, some bicrystals of WC are found in larger particles. Macro-crystalline WC is a desirable hardfacing material because of its erosion resistance and stability.
The third type of tungsten carbide used in hardfacing is cemented tungsten carbide, also known as sintered tungsten carbide. Cemented tungsten carbide comprises small particles of tungsten carbide (e.g., 1 to 15 microns) bonded together with cobalt. Cemented tungsten carbide is produced by mixing organic wax, tungsten carbide and cobalt powders, pressing the mixed powders to form a green compact, and “sintering” the composite at temperatures near the melting point of cobalt. The resulting dense cemented carbide can then be crushed and comminuted to form particles of cemented tungsten carbide for use in hardfacing.
As mentioned above, conventional hardfacing of milled-tooth bits usually comprises particles of tungsten carbide that are bonded to the steel teeth using a metal alloy. Most hardfacing on rock bits uses steel as the matrix (base), although other alloys may also be used. Steel or other alloys will generally be referred to as a binder alloy, and hardfacing compositions are typically applied from tube rods as disclosed in, for example, U.S. Pat. No. 5,250,355 issued to Newman et al.
More specifically, conventional hardfacing of milled-toothed bits usually include spherical cemented tungsten carbide particles larger than 60 mesh (e.g., greater than approximately 250 μm). The term “mesh” generally refers to the size of the wire mesh used to screen the carbide particles. For example, “60 mesh” indicates a wire mesh screen with sixty holes per linear inch, where the holes are defined by the crisscrossing strands of wire in the mesh. The hole size is determined by the number of meshes per inch and the wire size. The mesh sizes referred to herein are standard U.S. mesh sizes.
However, by using such large mesh sizes, “gaps” are left between the particles. As will be appreciated by those having ordinary skill, the larger the spherical particle, the larger the gap between particles. In order to provide sufficiently wear resistant compounds, a filler such as cast tungsten carbide or macrocrystalline tungsten carbide is typically used to fill the gaps between the larger spherical cemented tungsten carbide particles. In some cases, smaller spherical cemented carbide particles (e.g., less than 60 mesh) are also used to fill the gaps between then larger spherical cemented carbide particles. However, smaller tungsten carbide particles tend to break apart and dissolve into the steel matrix when exposed to welding temperatures. Even larger particles of 16 to 20 mesh can break apart during welding or the outer layers can dissolve.
A typical technique for applying hardfacing to the teeth on a rock bit is by oxyacetylene or atomic hydrogen welding. A welding “rod” or stick is typically formed as a tube of mild steel sheet enclosing a filler that mainly comprises carbide particles. The filler may also include deoxidizer for the steel, flux and a resin binder. The hardfacing is applied by melting an end of the rod on the face of the tooth. The steel tube melts as it is welded to the steel tooth and provides the matrix for the carbide particles. The deoxidizer alloys with the mild steel of the tube.
Although mild steel sheet is used when forming the tubes, the steel in the hardfacing on a finished a rock bit is a hard, wear resistant alloy steel. The conversion from a mild steel to the hard, wear resistant alloy steel occurs when the deoxidizers (which contain silicon and manganese) in the filler and tungsten, carbon, and possibly cobalt, from the tungsten carbide dissolve and mix with the steel during welding. There may also be some mixing with alloy steel from the teeth on the cone.
Advances in wear resistance of hardfacing are desirable to enhance the footage a drill bit can drill before becoming dull, and to enhance the rate of penetration of such drill bits. Such improvements translate directly into a reduction of drilling expenses. The composition of a hardfacing material and the physical structure of the hardfacing material applied to the surfaces of a drill bit are related to the degree of wear resistance and toughness. It is desirable to have a composition of hardfacing material that, when applied to wear surfaces, provides improved wear resistance and toughness while remaining relatively simple to apply to teeth.